The invention relates to azeotropes and methods of using azeotropes to clean substrates, deposit coatings and transfer thermal energy.
Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) have been used in a wide variety of solvent applications such as drying, cleaning (e.g., the removal of flux residues from printed circuit boards), and vapor degreasing. Such materials have also been used in refrigeration and heat transfer processes. While these materials were initially believed to be environmentally-benign, they have now been linked to ozone depletion. According to the Montreal Protocol and its attendant amendments, production and use of CFCs must be discontinued (see, e.g., P. S. Zurer, xe2x80x9cLooming Ban on Production of CFCs, Halons Spurs Switch to Substitutesxe2x80x9d, Chemical and Engineering News, page 12, Nov. 15, 1993). The characteristics sought in replacements, in addition to low ozone depletion potential, typically have included boiling point ranges suitable for a variety of solvent cleaning applications, low flammability, and low toxicity. Solvent replacements also should have the ability to dissolve both hydrocarbon-based and fluorocarbon-based soils. Preferably, substitutes will also be low in toxicity, have no flash points (as measured by ASTM D3278-89), have acceptable stability for use in cleaning applications, and have short atmospheric lifetimes and low global warming potentials.
Certain perfluorinated (PFCs) and highly fluorinated hydrofluorocarbon (HFCs) materials have also been evaluated as CFC and HCFC replacements in solvent applications. While these compounds are generally sufficiently chemically stable, nontoxic and nonflammable to be used in solvent applications, PFCs tend to persist in the atmosphere, and PFCs and HFCs are generally less effective than CFCs and HCFCs for dissolving or dispersing hydrocarbon materials. Also, mixtures of PFCs or HFCs with hydrocarbons tend to be better solvents and dispersants for hydrocarbons than PFCs or HFCs alone.
Many azeotropes possess properties that make them useful solvents. For example, azeotropes have a constant boiling point, which avoids boiling temperature drift during processing and use. In addition, when a volume of an azeotrope is used as a solvent, the properties of the solvent remain constant because the composition of the solvent does not change. Azeotropes that are used as solvents also can be recovered conveniently by distillation.
There currently is a need for azeotrope or azeotrope-like compositions that can replace CFC- and HCFC-containing solvents. Preferably these compositions would be non-flammable, have good solvent power, cause no damage to the ozone layer and have a relatively short atmospheric lifetime so that they do not significantly contribute to global warming.
In one aspect, the invention provides azeotrope-like compositions consisting essentially of hydrofluorocarbon ether and one or more organic solvents. The hydrofluorocarbon ether is represented by the general formula RfOCH3, where Rf is a branched or straight chain perfluoroalkyl group having 4 carbon atoms, and the ether may be a single compound or a mixture of the branched and straight chain ether compounds. The organic solvents are selected from the group consisting of: straight chain, branched chain and cyclic alkanes containing 6 to 8 carbon atoms; cyclic and acyclic ethers containing 4 to 6 carbon atoms; ketones having 3 carbon atoms; chlorinated alkanes containing 1, 3 or 4 carbon atoms; chlorinated alkenes containing 2 to 3 carbon atoms, alcohols containing 1 to 4 carbon atoms, partially fluorinated alcohols containing 2 to 3 carbon atoms, 1-bromopropane, acetonitrile, HCFC-225ca (1,1,-dichloro-2,2,3,3,3 pentafluoropropane) and HCFC-225cb (1,3-dichloro-1,1,2,2,3-pentafluoropropane). While the concentrations of the hydrofluorocarbon ether and organic solvent included in an azeotrope-like composition may vary somewhat from the concentrations found in the azeotrope formed between them and remain a composition within the scope of this invention, the boiling points of the azeotrope-like compositions will be substantially the same as those of their corresponding azeotropes. Preferably, the azeotrope-like compositions boil, at ambient pressure, at temperatures that are within about 1xc2x0 C. of the temperatures at which their corresponding azeotropes boil at the same pressure.
In another aspect, the invention provides a method of cleaning objects by contacting the object to be cleaned with one or more of the azeotrope-like compositions of this invention or the vapor of such compositions until undesirable contaminants or soils on the object are dissolved, dispersed or displaced and rinsed away.
In yet another aspect, the invention also provides a method of coating substrates using the azeotrope-like compositions as solvents or carriers for the coating material. The process comprises the step of applying to at least a portion of at least one surface of a substrate a liquid coating composition comprising: (a) an azeotrope-like composition, and (b) at least one coating material which is soluble or dispersible in the azeotrope-like composition. Preferably, the process further comprises the step of removing the azeotrope-like composition from the liquid coating composition, for example, by evaporation.
The invention also provides coating compositions consisting essentially of an azeotrope-like composition and a coating material which are useful in the aforementioned coating process.
In yet another aspect, the invention provides a method of transferring thermal energy using the azeotrope-like compositions of this invention as heat transfer fluids (e.g. primary or secondary heat transfer media).
The azeotrope-like compositions are mixtures of hydrofluorocarbon ether and one or more organic solvents which, if fractionally distilled, produce a distillate fraction that is an azeotrope of the hydrofluorocarbon ether and organic solvent(s).
The azeotrope-like compositions boil at temperatures that are essentially the same as the boiling points of their corresponding azeotropes. Preferably, the boiling point of an azeotrope-like composition at ambient pressure is within about 1xc2x0 C. of the boiling point of its corresponding azeotrope measured at the same pressure. More preferably, the azeotrope-like compositions will boil at temperatures that are within about 0.5xc2x0 C. of the boiling points of their corresponding azeotropes measured at the same pressure.
The concentrations of the hydrofluorocarbon ether and organic solvent or organic solvents in a particular azeotrope-like composition may vary substantially from the amounts contained in the composition""s corresponding azeotrope, and the magnitude of such permissible variation depends upon the organic solvent or solvents used to make the azeotrope-like composition. Preferably, the concentrations of hydrofluorocarbon ether and organic solvent in an azeotrope-like composition vary no more than about ten percent from the concentrations of such components contained in the azeotrope formed between them at ambient pressure. More preferably, the concentrations are within about five percent of those contained in the azeotrope. Most preferably, the azeotrope-like composition contains essentially the same concentrations of the ether and solvent as are contained in the azeotrope formed between them at ambient pressure. Where the concentrations of ether and organic solvent in an azeotrope-like composition differ from the concentrations contained in the corresponding azeotrope, the preferred compositions contain a concentration of the ether that is in excess of the ether""s concentration in the azeotrope. Such compositions are likely to be less flammable than azeotrope-like compositions in which the organic solvent is present in a concentration that is in excess of its concentration in the azeotrope. The most preferred azeotrope-like compositions will exhibit no significant change in the solvent power of the compositions over time.
The azeotrope-like compositions of this invention may also contain, in addition to the hydrofluorocarbon ether and organic solvent, small amounts of other compounds which do not interfere in the formation of the azeotrope. For example, small amounts of surfactants may be present in the azeotrope-like compositions of the invention to improve the dispersibility or solubility of materials, such as water, soils or coating materials (e.g., perfluoropolyether lubricants and fluoropolymers), in the azeotrope-like composition. Azeotropes or azeotrope-like compositions containing as a component 1,2-trans-dichloroethylene preferably also contain about 0.25 to 1 weight percent of nitromethane and about 0.05 to 0.4 weight percent of epoxy butane to prevent degradation of the 1,2-trans-dichloroethylene. Most preferably, such compositions will contain about 0.5 weight percent nitromethane and 0.1 weight percent of the epoxy butane.
The characteristics of azeotropes are discussed in detail in Merchant, U.S. Pat. No. 5,064,560 (see, in particular, col. 4, lines 7-48).
The hydrofluorocarbon ether useful in the invention can be represented by the following general formula:
Rfxe2x80x94Oxe2x80x94CH3xe2x80x83xe2x80x83(I)
where, in the above formula, Rf is selected from the group consisting of linear or branched perfluoroalkyl groups having 4 carbon atoms. The ether may be a mixture of ethers having linear or branched perfluoroalkyl Rf groups. For example, perfluorobutyl methyl ether containing about 95 weight percent perfluoro-n-butyl methyl ether and 5 weight percent perfluoroisobutyl methyl ether and perfluorobutyl methyl ether containing about 60 to 80 weight percent perfluoroisobutyl methyl ether and 40 to 20 weight percent perfluoro-n-butyl methyl ether are useful in this invention.
The hydrofluorocarbon ether can be prepared by alkylation of:
CF3CF2CF2CF2Oxe2x88x92, CF3CF(CF3)CF2Oxe2x88x92, C2F5C(CF3)FOxe2x88x92, C(CF3)3Oxe2x88x92 and mixtures thereof. The first three aforementioned perfluoroalkoxides can be prepared by reaction of: CF3CF2CF2C(O)F, CF3CF(CF3)C(O)F, and C2F5C(O)CF3 and mixtures thereof, with any suitable source of anhydrous fluoride ion such as anhydrous alkali metal fluoride (e.g., potassium fluoride or cesium fluoride) or anhydrous silver fluoride in an anhydrous polar, aprotic solvent in the presence of a quaternary ammonium compound such as xe2x80x9cADOGEN 464xe2x80x9d available from the Aldrich Chemical Company. The perfluoroalkoxide, C(CF3)3Oxe2x88x92, can be prepared by reacting C(CF3)3OH with a base such as KOH in an anhydrous polar, aprotic solvent in the presence of a quaternary ammonium compound. General preparative methods for the ethers are also described in French Patent No. 2,287,432 and German Patent No. 1,294,949.
Suitable alkylating agents for use in the preparation include dialkyl sulfates (e.g., dimethyl sulfate), alkyl halides (e.g., methyl iodide), alkyl p-toluenesulfonates (e.g., methyl p-toluenesulfonate), alkyl perfluoroalkanesulfonates (e.g., methyl perfluoromethanesulfonate), and the like. Suitable polar, aprotic solvents include acyclic ethers such as diethyl ether, ethylene glycol dimethyl ether, and diethylene glycol dimethyl ether; carboxylic acid esters such as methyl formate, ethyl formate, methyl acetate, diethyl carbonate, propylene carbonate, and ethylene carbonate; alkyl nitriles such as acetonitrile; alkyl amides such as N,N-dimethylformamide, N,N-diethylformamide, and N-methylpyrrolidone; alkyl sulfoxides such as dimethyl sulfoxide; alkyl sulfones such as dimethylsulfone, tetramethylene sulfone, and other sulfolanes; oxazolidones such as N-methyl-2-oxazolidone; and mixtures thereof.
Perfluorinated acyl fluorides (for use in preparing the hydrofluorocarbon ether) can be prepared by electrochemical fluorination (ECF) of the corresponding hydrocarbon carboxylic acid (or a derivative thereof), using either anhydrous hydrogen fluoride (Simons ECF) or KF.2HF (Phillips ECF) as the electrolyte. Perfluorinated acyl fluorides and perfluorinated ketones can also be prepared by dissociation of perfluorinated carboxylic acid esters (which can be prepared from the corresponding hydrocarbon or partially-fluorinated carboxylic acid esters by direct fluorination with fluorine gas). Dissociation can be achieved by contacting the perfluorinated ester with a source of fluoride ion under reacting conditions (see the methods described in U.S. Pat. No. 3,900,372 (Childs) and U.S. Pat. No. 5,466,877 (Moore), the description of which is incorporated herein by reference) or by combining the ester with at least one initiating reagent selected from the group consisting of gaseous, non-hydroxylic nucleophiles; liquid, non-hydroxylic nucleophiles; and mixtures of at least one non-hydroxylic nucleophile (gaseous, liquid, or solid) and at least one solvent which is inert to acylating agents.
Initiating reagents which can be employed in the dissociation are those gaseous or liquid, non-hydroxylic nucleophiles and mixtures of gaseous, liquid, or solid, non-hydroxylic nucleophile(s) and solvent (hereinafter termed xe2x80x9csolvent mixturesxe2x80x9d) which are capable of nucleophilic reaction with perfluorinated esters. The presence of small amounts of hydroxylic nucleophiles can be tolerated. Suitable gaseous or liquid, non-hydroxylic nucleophiles include dialkylamines, trialkylamines, carboxamides, alkyl sulfoxides, amine oxides, oxazolidones, pyridines, and the like, and mixtures thereof. Suitable non-hydroxylic nucleophiles for use in solvent mixtures include such gaseous or liquid, non-hydroxylic nucleophiles, as well as solid, non-hydroxylic nucleophiles, e.g., fluoride, cyanide, cyanate, iodide, chloride, bromide, acetate, mercaptide, alkoxide, thiocyanate, azide, trimethylsilyl difluoride, bisulfite, and bifluoride anions, which can be utilized in the form of alkali metal, ammonium, alkyl-substituted ammonium (mono-, di-, tri-, or tetra-substituted), or quaternary phosphonium salts, and mixtures thereof. Such salts are in general commercially available but, if desired, can be prepared by known methods, e.g., those described by M. C. Sneed and R. C. Brasted in Comprehensive Inorganic Chemistry, Volume Six (The Alkali Metals), pages 61-64, D. Van Nostrand Company, Inc., New York (1957), and by H. Kobler et al. in Justus Liebigs Ann. Chem., 1978, 1937. 1,4-diazabicyclo[2.2.2]octane and the like are also suitable solid nucleophiles.
The hydrofluorocarbon ethers used to prepare the azeotrope-like compositions of this invention do not deplete the ozone in the earth""s atmosphere and have surprisingly short atmospheric lifetimes thereby minimizing their impact on global warming. Reported in Table 1 is an atmospheric lifetime for the hydrofluorocarbon ether which was calculated using the technique described in Y. Tang, Atmospheric Fate of Various Fluorocarbons, M. S. Thesis, Massachusetts Institute of Technology (1993). The results of this calculation are presented under the heading xe2x80x9cAtmospheric Lifetime (years)xe2x80x9d. The atmospheric lifetimes of the hydrofluorocarbon ether and its corresponding hydrofluorocarbon alkane were also calculated using a correlation developed between the highest occupied molecular orbital energy and the known atmospheric lifetimes of hydrofluorocarbons and hydrofluorocarbon ethers that is similar to a correlation described by Cooper et al. in Atmos. Environ. 26A, 7, 1331 (1992). These values are reported in Table 1 under the heading xe2x80x9cEstimated Atmospheric Lifetime.xe2x80x9d The global warming potential of the hydrofluorocarbon ether was calculated using the equation described in the Intergovernmental Panel""s Climate Change: The IPCC Scientific Assessment, Cambridge University Press (1994). The results of that calculation are presented in Table 1 under the heading xe2x80x9cGlobal Warming Potentialxe2x80x9d. It is apparent from the data in Table 1 that the hydrofluorocarbon ether has a relatively short estimated atmospheric lifetime and relatively small global warming potential. Surprisingly, the hydrofluorocarbon ether also has a significantly shorter estimated atmospheric lifetime than its corresponding hydrofluorocarbon alkane.
Typical organic solvents useful in this invention include straight chain, branched chain and cyclic alkanes containing 6 to 8 carbon atoms (e.g., cyclohexane, methylcyclohexane, hexane, heptane and isooctane); cyclic or acyclic ethers containing 4 to 6 carbon atoms (e.g., t-butyl methyl ether, tetrahydrofuran and di-isopropyl ether); ketones containing 3 carbon atoms (e.g., acetone), chlorinated alkanes containing one, three or four carbon atoms (e.g., methylene chloride, 1,2-dichloropropane, 2,2-dichloropropane, t-butyl chloride, i-butyl chloride, 2-chlorobutane and 1-chlorobutane); chlorinated alkenes containing 2 to 3 carbon atoms (e.g., cis-1,2-dichloroethylene, 1,1,2-trichloroethylene, trans-1,2-dichloroethylene and 2,3-dichloro-1-propene); alcohols containing 1 to 4 carbon atoms (e.g., methanol, ethanol, 1-propanol, 2-propanol, i-butanol, t-butanol, 2-butanol), fluorinated alcohols having 2 to 3 carbon atoms (e.g., trifluoroethanol, pentafluoropropanol and hexafluoro-2-propanol), 1-bromopropane, acetonitrile and a 55 wt %/45 wt % mixture of HCFC-225ca and HCFC-225cb (respectively).
One or more of the organic solvents can be mixed with perfluorobutyl methyl ether to prepare the azeotropes and azeotrope-like compositions. Various examples of such azeotropes and azeotrope-like compositions are described in the Examples.
When nonhalogenated alcohols having 1 to 3 carbon atoms (i.e., methanol, ethanol, 1-propanol and isopropanol) are combined with the ether to make an azeotrope or azeotrope-like composition, the isomer composition of the ether may have some effect on the composition of the azeotrope. However, even in such mixtures, the boiling point of the azeotropes formed between the components are essentially the same.
Preferably, the azeotrope-like compositions are homogeneous. That is, they form a single phase under ambient conditions, i.e., at room temperature and atmospheric pressure.
The azeotrope-like compositions are prepared by mixing the desired amounts of hydrofluorocarbon ether, organic solvent or solvents and any other minor components such as surfactants together using conventional mixing means.
The cleaning process of the invention can be carried out by contacting a contaminated substrate with one of the azeotrope-like compositions of this invention until the contaminants on the substrate are dissolved, dispersed or displaced in or by the azeotrope-like composition and then removing (for example by rinsing the substrate with fresh, uncontaminated azeotrope-like composition or by removing a substrate immersed in an azeotrope-like composition from the bath and permitting the contaminated azeotrope-like composition to flow off of the substrate) the azeotrope-like composition containing the dissolved, dispersed or displaced contaminant from the substrate. The azeotrope-like composition can be used in either the vapor or the liquid state (or both), and any of the known techniques for xe2x80x9ccontactingxe2x80x9d a substrate can be utilized. For example, the liquid azeotrope-like composition can be sprayed or brushed onto the substrate, the vaporous azeotrope-like composition can be blown across the substrate, or the substrate can be immersed in either a vaporous or a liquid azeotrope-like composition. Elevated temperatures, ultrasonic energy, and/or agitation can be used to facilitate the cleaning. Various different solvent cleaning techniques are described by B. N. Ellis in Cleaning and Contamination of Electronics Components and Assemblies, Electrochemical Publications Limited, Ayr, Scotland, pages 182-94 (1986).
Both organic and inorganic substrates can be cleaned by the process of the invention. Representative examples of the substrates include metals; ceramics; glass; polymers such as: polycarbonate, polystyrene and acrylonitrile-butadiene-styrene copolymer; natural fibers (and fabrics derived therefrom) such as: cotton, silk, linen, wool, ramie; fur; leather and suede; synthetic fibers (and fabrics derived therefrom) such as: polyester, rayon, acrylics, nylon, polyolefin, acetates, triacetates and blends thereof; fabrics comprising a blend of natural and synthetic fibers; and composites of the foregoing materials. The process is especially useful in the precision cleaning of electronic components (e.g., circuit boards), optical or magnetic media, and medical devices and medical articles such as syringes, surgical equipment, implantable devices and prostheses.
The cleaning process of the invention can be used to dissolve or remove most contaminants from the surface of a substrate. For example, materials such as light hydrocarbon contaminants; higher molecular weight hydrocarbon contaminants such as mineral oils, greases, cutting and stamping oils and waxes; fluorocarbon contaminants such as perfluoropolyethers, bromotrifluoroethylene oligomers (gyroscope fluids), and chlorotrifluoroethylene oligomers (hydraulic fluids, lubricants); silicone oils and greases; solder fluxes; particulates; and other contaminants encountered in precision, electronic, metal, and medical device cleaning can be removed. The process is particularly useful for the removal of hydrocarbon contaminants (especially, light hydrocarbon oils), fluorocarbon contaminants, particulates, and water (as described in the next paragraph).
To displace or remove water from substrate surfaces, the cleaning process of the invention can be carried out as described in U.S. Pat. No. 5,125,978 (Flynn et al.) by contacting the surface of an article with an azeotrope-like composition which preferably contains a non-ionic fluoroaliphatic surface active agent. The wet article is immersed in the liquid azeotrope-like composition and agitated therein, the displaced water is separated from the azeotrope-like composition, and the resulting water-free article is removed from the liquid azeotrope-like composition. Further description of the process and the articles which can be treated are found in said U.S. Pat. No. 5,125,978 and the process can also be carried out as described in U.S. Pat. No. 3,903,012 (Brandreth).
The azeotrope-like compositions can also be used in coating deposition applications, where the azeotrope-like composition functions as a carrier for a coating material to enable deposition of the material on the surface of a substrate. The invention thus also provides a coating composition comprising the azeotrope-like composition and a process for depositing a coating on a substrate surface using the azeotrope-like composition. The process comprises the step of applying to at least a portion of at least one surface of a substrate a coating of a liquid coating composition comprising (a) an azeotrope-like composition, and (b) at least one coating material which is soluble or dispersible in the azeotrope-like composition. The coating composition can further comprise one or more additives (e.g., surfactants, coloring agents, stabilizers, anti-oxidants, flame retardants, and the like). Preferably, the process further comprises the step of removing the azeotrope-like composition from the deposited coating by, e.g., allowing evaporation (which can be aided by the application of, e.g., heat or vacuum).
The coating materials which can be deposited by the process include pigments, lubricants, stabilizers, adhesives, anti-oxidants, dyes, polymers, pharmaceuticals, release agents, inorganic oxides, and the like, and combinations thereof. Preferred materials include perfluoropolyether, hydrocarbon, and silicone lubricants; amorphous copolymers of tetrafluoroethylene; polytetrafluoroethylene; and combinations thereof. Representative examples of materials suitable for use in the process include titanium dioxide, iron oxides, magnesium oxide, perfluoropolyethers, polysiloxanes, stearic acid, acrylic adhesives, polytetrafluoroethylene, amorphous copolymers of tetrafluoroethylene, and combinations thereof. Any of the substrates described above (for cleaning applications) can be coated via the process of the invention. The process can be particularly useful for coating magnetic hard disks or electrical connectors with perfluoropolyether lubricants or medical devices with silicone lubricants.
To form a coating composition, the components of the composition (i.e., the azeotrope-like composition, the coating material(s), and any additive(s) utilized) can be combined by any conventional mixing technique used for dissolving, dispersing, or emulsifying coating materials, e.g., by mechanical agitation, ultrasonic agitation, manual agitation, and the like. The azeotrope-like composition and the coating material(s) can be combined in any ratio depending upon the desired thickness of the coating, but the coating material(s) preferably constitute from about 0.1 to about 10 weight percent of the coating composition for most coating applications.
The deposition process of the invention can be carried out by applying the coating composition to a substrate by any conventional technique. For example, the composition can be brushed or sprayed (e.g., as an aerosol) onto the substrate, or the substrate can be spin-coated. Preferably, the substrate is coated by immersion in the composition. Immersion can be carried out at any suitable temperature and can be maintained for any convenient length of time. If the substrate is a tubing, such as a catheter, and it is desired to ensure that the composition coats the lumen wall, it may be advantageous to draw the composition into the lumen by the application of reduced pressure.
After a coating is applied to a substrate, the azeotrope-like composition can be removed from the deposited coating by evaporation. If desired, the rate of evaporation can be accelerated by application of reduced pressure or mild heat. The coating can be of any convenient thickness, and, in practice, the thickness will be determined by such factors as the viscosity of the coating material, the temperature at which the coating is applied, and the rate of withdrawal (if immersion is utilized).